Conventional state-of-the-art lasers for outputting high-power blue-green or blue light having wavelengths in the 450-500-nm region are very costly, inefficient, and fragile. Prior-art devices for generating these wavelengths typically require many nonlinear conversion steps, including use of a tunable laser or optical parametric oscillator.
U.S. Pat. No. 6,288,835 titled “OPTICAL AMPLIFIERS AND LIGHT SOURCE” to Lars Johan Albinsson Nilsson is incorporated herein by reference. This patent describes single- or few-moded waveguiding cladding-pumped lasers, superfluorescent sources, and amplifiers, as well as lasers, including those for high-energy pulses, in which the interaction between the waveguided light and a gain medium is substantially reduced. This leads to decreased losses of guided desired light as well as to decreased losses through emission of undesired light, compared to devices of the prior art. Furthermore, cross-talk and inter-symbol interference in semiconductor amplifiers can be reduced. Also described are devices with a predetermined saturation power, and a single (transverse) mode optical fiber laser or amplifier in which the active medium (providing gain or saturable absorption) is shaped as a ring, situated in a region of the fiber's cross-section where the intensity of the signal light is substantially reduced compared to its peak value. The fiber can be cladding-pumped.
U.S. Pat. No. 4,867,558 titled “Method of remotely detecting submarines using a laser” that issued Sep. 19, 1989 to Leonard et al. and U.S. Pat. No. 4,893,924 titled “Method of remotely detecting submarines using a laser” that issued Jan. 16, 1990 also to Leonard et al. are both incorporated herein by reference. Leonard et al. describe monitoring subsurface water temperatures using a laser to detect subsurface waves in a body of water such as an ocean caused by a submarine. A pulsed laser beam is directed into the water to at least the depth of the thermocline and an analysis is made of the resultant Brillouin and Rayleigh backscatter components. Wavelength shifted Brillouin scatter is mixed with the unshifted Rayleigh scatter in a self-heterodyne manner for each volume element of illuminated water, and the frequency of the heterodyne signal is measured and converted into temperature. In those patents, the scheme is not directly detecting the submarine but instead is detecting the internal waves in the thermocline boundary in the seawater. The submarine's passage leaves ripples in the thermocline, which are subsequently detected by the system incorporating a laser.
U.S. Pat. No. 7,283,426 titled “Method and apparatus for detecting submarines” that issued to Grasso on Oct. 16, 2007 is incorporated herein by reference. Grasso describes detecting, tracking and locating submarines utilizing pulsed coherent radiation from a laser that is projected down through a water column, with particles in the water producing speckle from backscatter of the random particle distribution, with correlation of two closely time-spaced particle-based speckle patterns providing an intensity measurement indicative of the presence of a submarine. Subsurface submarine movement provides a subsurface wake which causes movement of particles such that two closely-spaced “snapshots” of the returns from particles in the same water column can detect particle movement due to the wake.
U.S. Pat. No. 5,270,780 titled “Dual detector LIDAR system and method” that issued to Moran et al. on Dec. 14, 1993 is incorporated herein by reference. This patent describes a light detection and ranging (LIDAR) system that uses dual detectors to provide three-dimensional imaging of underwater objects (or other objects hidden by a partially transmissive medium). An initial laser pulse is transmitted to known x-y coordinates of a target area. The photo signals returned from the target area from this initial pulse are directed to the low resolution, high bandwidth detector, where a preliminary determination as to the location (depth, or z coordinate) of an object in the target area is made based on the time-of-receipt of the return photo signal. A second laser pulse is then transmitted to the target area and the return photo signals from such second laser pulse are directed to the high resolution, narrow bandwidth detector. This high-resolution detector is gated on at a time so that only photo signals returned from a narrow “slice” of the target area (corresponding to the previously detected depth of the object) are received.
U.S. Pat. No. 5,504,719 titled “Laser hydrophone and virtual array of laser hydrophones” that issued to Jacobs on Apr. 2, 1996 is incorporated herein by reference. This patent describes a hydrophone or a virtual array of hydrophones for sensing the amplitude, frequency, and in arrays, the direction of sonic waves in water. The hydrophone employs a laser beam which is focused upon a small “focal” volume of water in which natural light scattering matter is suspended and which matter vibrates in synchronism with any sonic waves present. The vibration produces a phase modulation of the scattered light which may be recovered by optical heterodyne and sensitive phase detection techniques. The sonic waves are sensed at locations displaced from the focusing lenses.
U.S. Pat. No. 5,091,778 titled “Imaging LIDAR systems and K-meters employing tunable and fixed frequency laser transmitters” that issued to Keeler on Feb. 25, 1992 is incorporated herein by reference. Keeler describes a laser imaging system for underwater use that employs a wavelength-tunable laser. In particular, Keeler emphasizes the operation of the laser at blue wavelengths to optimize the performance in the open ocean.
U.S. Pat. No. 7,505,366 titled “Method for linear optoacoustic communication and optimization” that issued to Blackmon et al. on Mar. 17, 2009 is incorporated herein by reference. Blackmon et al. describe optical-to-acoustic energy conversion for optoacoustic communication from an in-air platform to an undersea vehicle. They describe directing a high-power laser at the ocean surface in order to generate acoustic waves (sound), wherein the sound is used as the communications signal to an underwater target receiver. Blackmon et al. assert that signals used in underwater acoustic telemetry applications are capable of being generated through a linear optoacoustic regime conversion process. They address the use of oblique laser beam incidence at an air-water interface to obtain considerable in-air range from the laser source to the water surface where the sound is formed to communicate to the undersea vehicle.
U.S. Patent Application Publication 2007/0253453 titled “Solid-state laser arrays using” published Nov. 1, 2007, and U.S. Patent Application Publication 2008/0317072 titled “Compact solid-state laser” published Dec. 25, 2008 both by Essaian and Shchegrov, are incorporated herein by reference. Essaian et al. describe a compact solid-state laser array for nonlinear intracavity frequency conversion into desired wavelengths using periodically poled nonlinear crystals. The crystals contain dopants such as MgO and/or have a specified stoichiometry. One embodiment includes a periodically poled nonlinear crystal chip such as periodically poled, MgO-doped lithium niobate (PPMgOLN), periodically poled, MgO-doped lithium tantalate (PPMgOLT), periodically poled, ZnO-doped lithium niobate (PPZnOLN), periodically poled, ZnO-doped lithium tantalate (PPZnOLT), periodically poled stoichiometric lithium niobate (PPSLN), and periodically poled stoichiometric lithium tantalate (PPSLT), periodically poled MgO- and ZnO-doped near-stoichiometric lithium niobate (PPMgOSLN, PPZnOSLN), or periodically poled MgO- and/or ZnO-doped near-stoichiometric lithium tantalate (PPMgOSLT, PPZnOSLT), for efficient frequency doubling of an infrared laser pump beam into the visible wavelength range. The described designs are said to be especially advantageous for obtaining low-cost green and blue laser sources. The use of such high-efficiency pumps and nonlinear materials allows scaling of a compact, low-cost architecture to provide high output power levels in the blue/green wavelength range.
What are needed are improved methods and apparatus for generating high-power pulses of infrared (IR) light of particular wavelengths and converting this light to blue-green and/or blue light. Also needed are systems capable of deep underwater communications, imaging, and other sensing using light obtained from a frequency-converted laser beam.